Thermal transfer systems have been used to generate prints from pictures which have been recorded from a color video camera or other electronic source, or which have been stored electronically from any source. Typically, the image is first separated into color separations, e.g. by passing the image through color filters and converting the respective color-separated images into electrical signals representing, for example, the cyan, magenta and yellow images. When the image is to be printed, these electrical signals are individually transmitted to a printer where each color is individually printed to generate a full color image. In one form of a thermal printer cyan, magenta and yellow dye-donor elements (sheets) are placed sequentially face-to-face with a dye-receiving element (sheet) and the mated dye-donor and receiver elements are inserted between a thermal printing head and a platen. The preferred method of thermal printing has heretofore employed the "forward" exposure process wherein the mated sheets are oriented so that the dye-donor sheet is adjacent the thermal printing head which applies heat to the back of the dye-donor sheet to drive the image dye in the forward direction, toward the receiver sheet. The thermal printing head has many heating elements which are sequentially actuated in response to the color signals. The process is then repeated for each of the other colors and a color hard copy is thus obtained which replicates the original image. One such process and apparatus is disclosed in U.S. Pat. No. 4,621,271, which is hereby incorporated by reference.
Other processes of obtaining thermal prints from electronic signals substitute one or more lasers for the thermal printing head. In such systems, the dye-donor sheet includes a material which strongly absorbs at the wavelength of the laser. When the dye-donor is irradiated with the beam of coherent light from the laser, the absorbing material converts the light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to transfer it to the receiver. The absorbing material may be present in a layer beneath the dye and/or it may be admixed with the dye. The laser beam is modulated by the electronic signals which are representative of the shape and color of the original image, so that each dye is heated only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. Further details of this process are found in British Patent No. 2,083,726A, the disclosure of which is also hereby incorporated by reference.
Published Japanese Patent Application No. 03/26595, published Feb. 5, 1991, teaches a laser thermal process in which the exposure is through a transparent receiver sheet, i.e. in the reverse direction toward a dye-donor element in which the light absorbing, heat producing layer is disposed as a discrete layer behind the dye layer. This publication alleges that the method taught therein provides higher print density or faster writing speed. In addition, it states that its method results in less adhesion failure between the dye dye-donor coating and the light absorbing layer than when forward printing through a transparent dye-donor element.
While thermal printing processes. employing both the forward and reverse exposure have been employed in systems employing a discreet layer of light-absorbing, heat-producing material beneath the dye layer, and employing forward exposure in systems employing a light-absorbing, heat-producing material admixed in the dye layer, these processes all share the problem that the final print density is highly susceptible to undesirable variability depending both upon the coating thickness uniformity of the dye layer and the optical density uniformity of the light-absorbing, heat-producing material. In other words, it has been found with all of the foregoing thermal systems, that variations in the coating thickness of the dye-donor dye layer will appear in the final image as undesirable variations in image density. Similarly, any variations in the optical density of the light-absorbing material also results in undesirable image density variations. With the necessity of maintaining very close tolerances in the manufacture of the dye-donor element to control both the optical density of the light absorbing material and the thickness of the dye layer, the cost of the dye-donor element is significantly increased, making the process more expensive and less commercially acceptable.
Still further, it has been found that, in prior art thermal processes employing thick dye layers, e.g., greater than approximately two microns thick, the relationship of power input to print density is non-continuous in that the change in print density resulting from a linear change in power input is not linear; the print density change can be disproportionate to the change in the power input. Such a non-linear density-to-power response makes it difficult, if not impossible, to obtain repeatable, satisfactory results.
According to my above-mentioned, co-pending application Ser. No. 996,989, entitled Laser Thermal Dye Transfer Using Reverse Exposure, it has been found that it is possible to overcome many problems of the prior art in providing a thermal image which is insensitive to donor dye thickness variations, thus permitting less strict manufacturing tolerances and costs, and resulting in a lower cost dye-donor material. At the same time, that application discloses a thermal printing process and apparatus which assures the production of an image in which the density varies linearly with the power level supplied to the radiation-generating device without unwanted discontinuities or variations in the power-to-density relationship which adversely affect the quality of the image produced.
However, the process and apparatus disclosed in that application still employ donor elements or sheets upon which the dye is provided for transfer to a receiver element during the writing process. Such donor elements are consumable during the thermal printing process and constitute a significant portion of the cost of the process.